1. Field of the Invention
The present invention relates to a circuit flat cable, and in particular to a composite flexible circuit planar cable that combines a regular flexible planar cable, such as a flexible flat ribbon cable that is often referred to as flexible flat cable, and a regular circuit board.
2. The Related Arts
Processes for manufacturing a flexible circuit board are often classified as (1) forming a flexible board through image transfer, etching conductive layer, and laminating insulation layer, (2) directly printing conducive layer on a flexible board of insulation base material, and (3) forming a standard planar cable by stretching a plurality of parallel straight copper wires with a wind-up machine and laminating insulation layer. Dividing by functionality, there can be carrier boards, planar cables, or flexible circuit board featuring the functions of both carrying board and planar cable. Thus, a flexible circuit board is also referred to in different terms, such as flexible printed circuit board and flexible flat cable. These terms are actually directed to the same product. Based on the classification made according to manufacturing cost, a flexible board made through etching is the most expensive one and printing is the next expensive one, and the standard flexible flat cable is the cheapest one.
With increasing competition and mass production, to further reducing the manufacturing cost, the designs of internal components must be increasingly simplified. And, interconnection between modules can be done with a flexible flat cable to provide a path for signal transmission, and more importantly, to lower down the cost. However, the regular flexible flat cable often generates high frequency and high energy electromagnetic waves when transmitting signals, and also, the signal transmitted by the cable is easily subjected to interference caused by external high frequency noises, leading to distortion of the signal. Further, a flexible flat cable or a circuit board, when extending through a hole that is often put in movement, may generate high voltage static electricity. Under such a condition, fast and effective grounding is very vital.
To handle such drawbacks, a known solution is to coating a metal shielding layer on a surface of the flexible circuit board to shield external electromagnetic noises. However, such a metal shielding layer is effective in isolating the interference caused by external electromagnetic noises, but the metal shielding layer so coated is not effectively connected to ground of an electronic device, so that the performance of the metal shielding layer in shielding electromagnetic noises and eliminating static electricity is not good enough.
Thus, internal modules of a conventional electronic device often commonly use a conventional flexible circuit board made with an etching process as signal connection for easy grounding. However, the cost of the conventional flexible circuit board is much higher than the standard flexible flat cable. Further, one of the reasons that the standard flexible flat cable does not possess the function of electromagnetic noise shielding is that the manufacturing process is carried out by stretching and laminating copper wires so that the copper wires are of small width, making it difficult to control the impedance thereof. Thus, it is a major challenge for the industry to use regular flexible flat cable to replace the usage area of a fraction of the conventional flexible circuit board for reduction of cost and to provide effective shielding against electromagnetic noise and to provide impedance control for signal, and at the same time featuring a conductive connection structure for elimination of static electricity. Further, in some product applications, extension is made through a small hole or a bore of a hinge and tens of thousands of times of tests must be passed for bending durability. This is no exemption for applications of bundle like structure that is formed by partial slitting.
In addition, in an attempt of applying interconnection formed with standard flexible flat cable to provide a path for signal transmission for the purposes of lowering cost, when the positions of the corresponding signal terminals of the modules to be connected are different from each other, electrical connection will not be established between the positions of signal terminals and the positions of corresponding signal terminals.